In the EU's draft withdrawal agreement, published in February, the three options for avoiding a hard border between Northern Ireland and the Irish Republic were: a comprehensive trade deal; a bespoke, most likely technological solution; or "full regulatory alignment" between the two territories.
"Also in the future, I suppose we try to keep interconnections between European Union and UK-based financial sector", Katainen said.
Prime Minister Theresa May said this was unacceptable.
Overall, he added, "it is time to face up to the hard facts".
This prompted an angry response from UKIP MEP David Coburn, who retoted that it was a "British issue".
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Trump is expected to arrive on Tuesday for his first visit to California as president to see the wall's prototypes . Democrats already have their 2018 sights set on California, targeting seven vulnerable GOP-held seats in the state.
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Australia, who are fourth in the 10-stop competition's standings, topped their group on Saturday with two wins and a draw. Earlier in the day, Fiji edged South Africa 15-12 and Kenya beat the United States 24-19 in the semi-finals.
"The UK is choosing to leave the union, the single market and the customs union, we have noted that".
The UK and the European Union are set to negotiate a deal later this month that will largely keep things as they are until 2020, but a number of companies are already concerned about the long-term impact on the industry that employs over 800,000 people and generates turnover of £77.5 billion.
Mr Juncker singled out the Irish border issue and said the United Kingdom should come forward with "concrete solutions" to address it if it wanted to avoid the back-stop of keeping full alignment with single market rules, agreed in December.
"I would rather have preferred Britain not to have chose to leave the European Union, but anyone who leaves the European Union has to know, frankly, what this means", he said. Do you accept that, or do you then put in a guard to stop them doing that.
His European Parliament motion calls for an "association agreement" to deal with the future UK-EU relationship.
"I fully respect the Chancellor's competence in defining what is in the UK's interest but he must allow us to define what is in the EU's interest", he said.
A spokeswoman for US President Donald Trump refused to blame Russian Federation for an attack on what she called Washington's "closest ally" but Secretary of State Rex Tillerson said he was confident in May's assessment of Moscow's role and promised that there should be "serious consequences".